Liquid crystal display devices have the advantage of being thin in thickness, light in weight, and low in power consumption among various display devices, and have recently been widely used in various fields such as TVs (televisions), monitors, and portable terminals instead of CRTs (cathode-ray tubes).
Conventionally, in order to protect privacy during use of portable terminals such as cellular phones, various anti-peeking techniques have been proposed. One of them is a technique that utilizes the viewing angle characteristics of liquid crystals.
(a) of FIG. 10 is a diagram showing a γ (gamma) characteristic of a typical VA (vertical alignment) liquid crystal panel, and (b) of FIG. 10 is a diagram showing a gamma characteristic obtained by using a multi-gamma technique.
In (a) of FIG. 10, γ11 indicates a gamma curve obtained when the liquid crystal panel is looked squarely at, and γ12 indicates a gamma curve obtained when the liquid crystal panel is looked obliquely at.
Further, in (b) of FIG. 10, γ13 indicates a gamma curve obtained when the liquid crystal panel is divided into first and second regions and the first and second regions are looked squarely at, and γ14 indicates a gamma curve obtained when the first region is looked obliquely at, and γ15 indicates a gamma curve obtained when the second region is looked obliquely at.
It should be noted that looking squarely or obliquely at the liquid crystal panel means looking squarely or obliquely at a display screen of the liquid crystal panel. Further, in the following description, the term “during square viewing” refers to the time when the liquid crystal panel is looked squarely at, and the term “during oblique viewing” refers to the time when the liquid crystal panel is looked obliquelyat.
As shown in (a) of FIG. 10, a typical VA liquid crystal display device shows a phenomenon (excess brightness) of a gray level becoming higher in luminance when looked obliquely at.
That is why there is a technique (multi-gamma) for adjusting luminance in adjacent pixels to reduce excess brightness. Conventionally, a technique for effecting privacy by applying the multi-gamma technique has been known.
The technique is a technique for, with the liquid crystal panel having its display region divided into first and second regions, applying a normal gamma voltage to the first region and applying a multi-gamma voltage to the second region.
According to the technique, as shown in (a) of FIG. 10, the first region and the second region are substantially equal in luminance to each other when the liquid crystal panel is looked squarely at, but differ in luminance from each other when the light crystal panel is looked obliquely at. Therefore, privacy is effected by utilizing a phenomenon of a checkered pattern appearing when the liquid crystal panel is looked obliquely at and making it hard to see characters, etc. that are displayed in the first region when the liquid crystal panel is looked squarely at.
Patent Literature 1 discloses a technique for preventing peeking by applying the foregoing technique to display, during oblique viewing in a narrow viewing angle mode, a composite image obtained by combining a primary image and a secondary image that is different from the primary image.
(a) of FIG. 11 is a diagram showing a relationship between pixel data of pixels adjacent to each other in the first region of Patent Literature 1 and the averages of luminances during square viewing and during oblique viewing, and (b) of FIG. 11 is a diagram showing a relationship between pixel data of pixels adjacent to each other in the second region of Patent Literature 1 and the averages of luminances during square viewing and during oblique viewing.
In order to cause a checkered pattern to be displayed when the liquid crystal panel is looked obliquely at, Patent Literature 1 sets up pixel data for each separate pixel as shown in (a) and (b) of FIG. 11. That is, in the first region, the pixel data of all the pixels take on a value of “189”, and in the second region, the pixel data of upper left and lower right pixels of four pixels adjacent to each other take on a value of “0” and the pixel data of the remaining two pixels take on a value of “255” so that the pixel data of adjacent pixels in the second region take on different values from each other.
In this case, the averages of luminances during square viewing in the first and second regions are both 50%. Therefore, when the liquid crystal panel is looked squarely at, the first and second regions look the same way. Meanwhile, during oblique viewing, the first region has an average luminance factor of 39% and the second region has an average luminance factor of 20%. The difference in luminance between the average luminance factors is seen as a notable difference in luminance when the liquid crystal panel is looked obliquely at. As a result, a composite image obtained by combining a checkered pattern with the primary image is seen from an oblique angle. Therefore, peeking is prevented.
Further, Patent Literature 2 discloses a technique for making an image seen from an oblique angle unclear by making transmittance during square viewing and transmittance during oblique viewing different from each other by partially switching the orientation of liquid crystals in each pixel between a narrow viewing angle and a wide viewing angle.
Patent Literature 3 discloses a technique for, when a user other than a user who has logged in the device has been detected, changing the output state of an image or a sound so that it is difficult to recognize the image or the sound.